The present invention relates to a method for preparing a film having preferential one-sided cling.
Olefinic polymers, particularly polymers of ethylene, are employed in a variety of end-use applications including the preparation of wrapping films. In conventional wrapping processes, e.g., spin or rotary stretch wrapping techniques, the polymer film is wound under tension around a package by either rotating the package on a turntable or by winding the film around a stationary package. In such applications, it is generally desirable for the overlapping portions of the film to adhere to each other thereby self-sealing the wrap without the need for an adhesive or heat sealing operation.
Conventionally, to impart the necessary tackiness or "cling" to the film, the polymers are blended with a tackifying agent such as polybutene, a terpene resin, hydrogenated resin and resin esters and a film subsequently prepared from the resulting blend. See, for example, U.S. Pat. No. 3,025,167; Swedish Patent Application No. 7807733-6 and European Patent Application No. 0011930. Unfortunately, the mere blending of such tackifying agents within an olefinic polymer imparts equal tack to both surfaces of the film prepared therefrom. Therefore, upon contact of the wrapped packages, the individual packages stick or cling to one another and, upon parting, there is a tendency for the wrappings to be disturbed or torn.
Alternatively, French Patent Application No. 2,031,801 describes a method for preparing a polymeric film having a tacky surface by coating a continuous polyethylene film with a layer of a copolymer of ethylene and vinyl acetate blended with polyisobutene, butyl rubber, polypropylene or atactic polybutene. While a film having cling primarily on one side may be prepared by coating only one surface of the polymeric film with said copolymer, the equipment and additional processing required to prepare the laminated film makes the film economically unattractive.
Alternatively, British Patent Specification No. 1,578,063 teaches that a polymeric film having one surface substantial more clingy than the other surface can be prepared by forming a film from a blend of polyisobutylene and polyethylene and thereafter treating one surface of the film with corona discharge. Similarly, U.S. Pat. No. 4,425,268 teaches that a composition comprising from 40 to 90 weight percent of a high molecular weight copolymer of ethylene and vinyl acetate, from 8 to 55 weight percent of a linear copolymer of ethylene and a higher alkene and from 0.5 to 6 weight percent of a tackifier can be extruded to produce a plastic film having one-sided cling by passing one side only of the film through a corona discharge zone. Unfortunately, the corona discharge treatment of the film requires the purchase and installation of additional process equipment thereby necessitating the expenditure of substantial time and capital.
In view of these deficiencies in polymeric films of the prior art, it remains highly desirable to provide a polymeric film having one surface tackier than the other surface which polymeric film possesses excellent physical properties.